1. Field of the Invention
The present invention relates to a color filter to be used for a liquid crystal display that may be used for both a transmission type and reflection type display.
2. Description of the Related Art
A liquid crystal display is used today for various uses such as a notebook-sized personal computer, portable type information terminal, desk-top monitor and digital camera by taking advantage of its characteristics such as a lightweight and thin size, and low power consumption. The liquid crystal display using a backlight is required to improve energy efficiency of the backlight for reducing power consumption as well as to enhance transmittance of the color filter. While the transmittance of the color filter is advancing year after year, power consumption cannot be expected to be largely decreased by improving transmittance of the color filter.
Accordingly, a reflection type liquid crystal display has been developed in recent years in order to eliminate the need of the backlight that consumes a large amount of electric power. Released is a reflection type liquid crystal display that has realized an energy consumption as small as 1/7 of energy consumption of a transmission type liquid crystal display (“Flat-Panel Display”, Supplement Edition of Nikkei Micro-Device, 1998, p126).
Although it is advantages of the reflection type display over the transmission type display that power consumption is small and outdoor viewing is excellent, the display panel becomes dark when a sufficient intensity of light is not ensured in the environment accompanying extremely poor viewing. Accordingly, a proposed liquid crystal display designed for improving viewing even in a dark environment comprises (1) a partially transmission type and partially reflection type liquid crystal display having a cut portion in a part of a reflection film together with providing a backlight (based on a so-called semi-transmission semi-reflection type display method; see as a reference Fine Process Technology Japan '99, Textbook of Specialized Technology Seminar), and (2) a display device having a front-light.
FIG. 3 is a schematic drawing of a conventional semi-transmission type liquid crystal display. FIG. 3 shows a transmission mode that displays by taking advantage of a backlight source 3, and a reflection mode that displays by taking advantage of a natural light. This type of the color liquid crystal display comprises a transmission area 8 and reflection area 7, and the color in the transmission area is displayed for transmission display while the color in the reflection area is displayed for reflection display. While the backlight passes through the transmission area of the color filter once (denoted by an arrow 6) for transmission display, the natural light passes through the reflection area of the color filter twice, or as an incident light 4 and a reflecting light 5 reflected with a reflection layer 2. In other words, since the number of passage through the color filter is different between the transmission type display and reflection type display, the displayed color density, or the color purity and brightness are largely different between the transmission type display and reflection type display when the same color material 1 is used for the transmission area and reflection area. Since the light sources are a backlight 3 and natural light 4, respectively, in the transmission type display and in the reflection type display, the color purity as well as the color tone are different between the transmission type display and reflection type display.
It may be contemplated to constitute the transmission area and reflection area with different color materials 1 and 1′, respectively, as shown in FIG. 5 in order to obtain the same display color in both the transmission area and reflection area. However, the manufacturing cost of the liquid crystal display seems to be increased by using a currently prevailing photolithographic method since six times of coating using three colors are required.
Japanese Unexamined Patent Application Publication No. 2001-33778 discloses to form a spacer in the reflection area for changing the thickness of the color layers between the transmission area and reflection area, in order to make the color density (color repeatability) equal between the transmission area and reflection area. FIG. 6 is a schematic cross section of a color filter for a semi-transmission type liquid crystal display having a construction known in the art. A transparent resin layer 14 is formed in the reflection area 7, and the color layer 1 of the reflection area 7 is thinner than the color layer 1 of the transmission area 8. However, changes of the color tone that arise due to the difference of the light sources, or the backlight in the transmission display and natural light in the reflection display, cannot be corrected merely by changing the thickness of the color materials. In other words, while large differences in the color purity and brightness may be eliminated by thinning the thickness of the color layer of the reflection area 7, the monochromatic color tone of each red, green and blue color in the reflection type display becomes different from that in the transmission type display, thereby giving a disharmony in vision of the reflection type display and transmission type display.
In FIGS. 3, 5 and 6, the reference numeral 9 denotes a pixel area.
A light-hole type color filter has been proposed for solving the problems as hitherto described and for cheaply providing a color filter having less differences of the brightness and color tones between the transmission type display and reflection type display, wherein a transparent area is provided in the reflection area as disclosed in Japanese Unexamined Patent Application Publication No. 2000-111902. Since only one time of processing is necessary for one color, it may be manufactured by the same number of manufacturing steps as in the conventional color filters, thereby avoiding the manufacturing cost from being increased.
FIG. 1 shows the structure (cross sectional structure) of the light-hole type color filter. A transmission area 8 and reflection area 7 are formed in a pixel for each one color in this color filter. Both areas may be present in one pixel, or either one of them may be present in one pixel and the other area may be present in a different pixel to form the both areas in a plurality of pixels. The substrate on which a reflection layer 2 is formed may be either a substrate at the color filter side, or a substrate opposed to the color filter. The area in which the reflection film 2 is formed serves as a reflection area 7 within the pixel area 9 when the reflection film is formed at the color filter side, and the area having no reflection film 2 within the pixel area 9 serves as a transmission area 8. When the reflection film is formed on a substrate opposed to the color filter, on the other hand, the pixel area corresponding to the reflection film forming area of the substrate serves as the reflection area 7, while the pixel area corresponding to the area where the reflection film 2 is not formed on the substrate serves as the transmission area 8. The reflection area 7 contains a transparent area 10 and colored area 11. The transparent area 10 actually means an area where no color layer 1 is formed.
However, it is a problem of the light-hole type color filter that a step height is formed on the surface of the color filter as a result of providing a transparent area having no color layer. One of the fundamental characteristics that seriously affect the display performance of the liquid crystal display is a cell gap. Since the step height on the surface of the color filter is directly reflected on the change of the cell gap, it is preferable that the step height is as small as possible. In addition, since a larger step height may adversely affect planarity of the color filter to arise inconvenience for alignment treatment of the layer, defective display may appear when the filter is integrated into a liquid crystal panel.
The step height between the color area and transparent area in the reflection area may be planarized to a certain extent by coating an overcoat layer. However, when a pigment comprising a quinacridone derivative represented by pigment red 209 as disclosed in Japanese Unexamined Patent Application Publication No. 2000-249824 is used as a principal pigment, the step height increases due to the increased thickness of the pixel in the effort for improving the color purity and color characteristics of the color filter, making it difficult to fill the step even by coating the overcoat.
Control of dimensions of the transparent area is quite important in the light-hole type color filter. Therefore, poor processing performance results in low yield of the filter, thereby increasing the manufacturing cost of the color filter.